Abnormal interjoint coordination is a hallmark of neuropathologies such as stroke and spinal cord injury, and there is evidence that changes in stretch reflex function may contribute to this discoordination. Understanding the significance of these changes and how they might be reversed or reduced through rehabilitation is best done in the context of the unimpaired state. However, there is little information regarding the role of stretch reflexes in the unimpaired control of multijoint movement and posture. Rather, most human studies have focused on the stretch response to single joint perturbations, which allow for unambiguous regulation of muscle length, but do not address the complexities of real perturbations and real movements involving multiple muscles acting in multiple degrees of freedom. This purpose of this grant is to examine stretch reflex contributions to multijoint coordination in the unimpaired human arm. Specifically we will determine if the muscle activation patterns generated in response to whole limb perturbations arise from neurally-mediated, intermuscular (heteronymous) connections or if they simply represent the autogenic (homonymous) responses of individual muscles linked via the skeleton. In addition, the functional consequences of these actions will be addressed by quantifying the mechanical impact of any observed reflex actions. Our Aims are: 1) To examine the heteronymous reflexes connecting the elbow and shoulder by applying single joint perturbations to a single joint, and recording the changes in muscle activity elicited at both joints. These results will provide a "model-free" assessment of heteronymous linkages. 2) To examine the relative contributions of homonymous and heteronymous stretch reflex pathways in coordinating the response to whole limb perturbations. This will be investigated using a 3DOF robot to perturb limb posture and a 3D biomechanical model to estimate the resulting muscle length changes. Changes in muscle activation that are not directly linked to homonymous length changes are likely to represent the output of heteronymous pathways. 3) To examine the influence of reflexively evoked changes in muscle activity on the regulation of multijoint mechanics. The first two Aims are important for assessing the existence and connectivity of stretch reflex responses, but alone do not establish their functional consequences. The latter requires an assessment of how reflex responses influence the mechanical properties of a limb. This will be accomplished using nonlinear system identification techniques to estimate stretch reflex contributions to multijoint impedance. This study will be the first to investigate the role of multijoint stretch reflexes for coordinating arm posture in the 3DOF relevant to normal function. Our results will provide the essential basis for our further investigations into abnormal reflex function following stroke and will elucidate the degree to which spinal mechanisms should be targeted by rehabilitation interventions.